Corrugated board conventionally is prepared by a process known as the Stein-Hall process. As is generally described in U.S. Pat. No. 2,102,937 (issued to J. V. Bauer), the Stein-Hall process employs a corrugating adhesive to bond a corrugated paper "medium," such as a roll or strip, to a liner board on one or both sides of the corrugated medium. Adhesives that are used in conjunction with the Stein-Hall process long have been known, and such adhesives generally comprise an aqueous emulsion of raw starch; caustic; pasted modified or unmodified starch; and a cross-linking agent. The raw starch serves as a binder in the finished corrugated board, while the pasted starch and cross-linking agent form a tacky composition that holds the plural layers of green corrugated board together before the primary starch adhesive has set. The cross-linking of the pasted starch is further thought to impart suspending power and to affect the viscosity of the corrugating adhesive during storage and application. For these reasons, the prior art has taught that the use of pasted starches in corrugating adhesives is necessary.
Known corrugating adhesives suffer from a number of drawbacks. For example, the speed of the machinery used to prepare the corrugated board sometimes is limited by the rheological properties of the adhesive. During manufacture of corrugated board, the corrugating adhesive typically is spread across the liner board or the corrugated paper medium with a spreader knife or metering roller. It has been observed that conventional adhesives undergo substantial shear thinning when they are spread too quickly, thus leading to problems in applying the adhesive to a corrugating medium in conventional corrugating equipment. The shear thinning thus may serve to limit the speed of the corrugating equipment, and thus may limit the attainable output of corrugated board.
Another drawback relates to the green bonding strength of conventional corrugating adhesives and, more specifically, to the rate at which the tack of the adhesive increases when the corrugated board is in the green state. Typically, corrugated board is processed and handled before the adhesive has fully dried, the adhesive thus being in the green state. If the adhesive has not become tacky quickly enough, then the corrugated board will delaminate during the processing operations that follow the bonding operation. The rate of increase of tackiness of known adhesives thus may be a further limiting factor in the rate of manufacture of corrugated board.
It is believed that the rheological instability of conventional corrugating adhesives can be attributed to the presence of pasted starches in the adhesive composition. In recognition of this belief, recent prior art has taught to replace a portion or all of the pasted starch in the corrugating adhesive with hemicellulose, a plant derivative obtained from a hemicellulose-containing plant source such as corn hulls. For example, U.S. Pat. Nos. 5,358,559 and 5,503,668 each purport to provide a corrugating adhesive composed of starch and hemicellulose, the hemicellulose being derived from corn hulls. Similarly, U.S. Pat. No. 5,777,005 purportedly provides a corrugated starch based on a combination of corn hulls and tapioca fiber. Each of these patents teaches to blend corn hulls with starch and to extract hemicellulose in-situ from the corn hulls to provide a hemicellulose-based adhesive composition.
While such recent prior art purports to provide hemicellulose-containing corrugating adhesives, these adhesives are unsatisfactory in some respects. For example, the rate of increase of tack of these adhesives when in green state is unsatisfactory, thus potentially leading to delamination problems if the corrugating operation is run at too high a speed. Thus, while the rheological stability of such corrugating adhesives may be improved as compared with those of corrugating adhesives based on pasted starches, these adhesives still are of limited value in increasing the rate of production of corrugated board.
It is a general object of the invention to provide a corrugating adhesive that is more rheologically stable than conventional corrugating adhesives. Another general object is to provide a corrugating adhesive that has excellent bonding properties, both in the green state with respect to the rate of increase of tack and in the final corrugated board product. Moreover, while the bonding strengths of conventional corrugated adhesives are satisfactory, it is another general object to provide a corrugating adhesive that provides superior bonding strength in a corrugated board.